But is the overmind really dead? My hunch is, just like Tassadar, he has never "tasted death."

I think they merged. After all, Tassadar says that he has access to the memories of the Overmind. Perhaps they created a "good" hybrid (a super-archon) and thereby completed the xel'naga cycle. It is possible that the Overmind embedded itself in the crust of the planet for this very reason - yet another way of circumventing Amon's orders. The great Overdar/Tassamind is waiting for the opportune time to reveal himself. He will probably help defeat Amon, either single-handedly or by creating several more versions of himself using other willing protoss and zerg.

If you ask me, they should find a way of resurrecting the Overmind for Starcraft III. Once Amon is dead (I expect that will be the case in LotV), who will be the "bad guy" in the next game? Seeing a return of the Overmind would be awesome. Maybe have the Swarm split into multiple factions, with the Overmind leading one and Kerrigan leading the other (or Zagara if Kerrigan doesn't survive LotV).

And then there's still Niadra. She is quite clearly going to play a major role in either LotV or SC3.

As much as I love The Overmind, I doubt it will be resurrected. I sure hope more is expanded on it, and it's corpse is revisited in LOTV (perhaps even the Overmind in spirit form). My belief is that, IF it was to be resurrected, The Overmind would be an enemy again. A slave to Amon to use to disrupt Kerrigan's control over the Swarm.

If you ask me, they should find a way of resurrecting the Overmind for Starcraft III. Once Amon is dead (I expect that will be the case in LotV), who will be the "bad guy" in the next game? Seeing a return of the Overmind would be awesome. Maybe have the Swarm split into multiple factions, with the Overmind leading one and Kerrigan leading the other (or Zagara if Kerrigan doesn't survive LotV).

And then there's still Niadra. She is quite clearly going to play a major role in either LotV or SC3.

This is what's likely going to happen. Niadra will be fked over by the protoss. And when Amon is defeated and everybody is celebrating, the UED main battle fleet appears :p There you have it. SC3.

Then he got killed by a carrier that no body could magiclly stop even though there is billions of zerg........

Most of the zerg weren't in the area (having been thrown into confusion by Zeratul assassinating two cerebrates a short while before.) And it was pretty much the Protoss flagship (far stronger than a normal carrier).